1. Field of the Disclosure
Embodiments disclosed herein relate to shaker screens for vibratory sifters. More specifically, embodiments disclosed herein relate to shaker screens for vibratory sifters that are pre-tensioned. More specifically still, embodiments disclosed herein relate to apparatuses and methods for using gyratory sifters including pre-tensioned shaker screens including rigid external extensions.
2. Background Art
Generally, sifters include a class of vibratory devices used to separate sized particles, as well as to separate solids from liquids. Sifters are used to screen, for example, feed material, plastic resins, and powders during industrial sorting and/or manufacturing operations.
Because sifters may be in continuous use, repair operations, and associated downtimes need to be minimized as much as possible. Often, the filter screens of sifters, through which sized materials or liquids are separated, wear out over time and subsequently require replacement. Therefore, sifter filter screens are typically constructed to be removed and replaced. While there are numerous styles and sizes of filter screens, they generally follow similar design.
Typically, filter screens include a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids or sized material therethrough, while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are flat or slightly arched, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. The pyramid-shaped channels may provide additional surface area for the sized solids or fluid-solid separation process to take place while acting to guide solids along their length toward the end of the sifter from where they are disposed.
A typical sifter filter screen includes a plurality of hold-down apertures at opposite ends of the filter screen. These apertures, preferably located at the ends of the filter screen that abut walls of the sifter, allow hold down retainers of the sifter to grip and secure the filter screens in place. However, because of their proximity to the working surface of the filter screen, the hold-down apertures must be covered to prevent solids in the material passing therethrough from bypassing the filter mesh through the hold-down apertures. To prevent such bypass, an end cap assembly is placed over each end of the filter screen to cover the hold-down apertures.
In alternate sifters, a filter mesh may be stretched over a frame that is an integral part of the sifter body. Thus, such filter meshes may become tensioned as they are disposed within the sifter. Such methods of attaching filter mesh may involve time consuming filter mesh changing operations, as the old filter mesh must be removed, and a new filter mesh must be tensioned within the sifter body. Removing and then reattaching a filter mesh may involve substantial manual labor, and require significant sifter downtime.
In certain sifter screens, the screen includes a frame with a filter screen attached to the top of the screen. The screen is generally a flat screen with a plurality of location tabs that extend from the base of the frame that help guide the frame into position within the sifter. The lateral extension of the location tabs then press against the body of the sifter, thereby creating a desired tension across the surface of the screen. However, such screens are prone to design failure, because the tensioning of the frame may cause bowing and otherwise cause breaks to form in the body of the frame. Furthermore, the location tabs often wear out over time, decreasing the tension on the frame, and decreasing the sealing efficiency of the screen to the sifter. As sealing efficiency decreases, sized particles may slip though spaces along the screen edge, leading to product loss, further stress points along the screen that may lead to premature screen failure, and otherwise result in the requirement to change the filter mesh more frequently.
Typically, screens used with sifters are placed in a generally horizontal fashion on a substantially horizontal bed or support structure located within a basket in the sifter. The screens themselves may be flat, nearly flat, corrugated, depressed, and/or contain raised surfaces. The basket in which the screens are mounted may be inclined towards a discharge end of the sifter. The sifter imparts a rapidly reciprocating motion to the basket and the screens. A source material, from which particles are to be separated, is poured onto a back end of the vibrating screen. The material generally flows toward the discharge end of the basket. Large particles that are unable to pass through the screen remain on top of the screen, and move toward the discharge end of the basket where they are collected. Smaller particles and/or fluid pass through the screen and collect in a bed, receptacle, or pan therebeneath.
In some sifters, a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth or mesh. Layers of cloth or mesh may be bonded together and placed over a support, multiple supports, a perforated plate, or an apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support, and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated to create a flow of trapped solids on top surfaces of the screen for removal and disposal thereof. The fineness or coarseness of the mesh of a screen may vary depending upon the operational requirements of a specified sifting operation.
Replacing un-tensioned screens and filter mesh used in sifters generally is a time consuming and inefficient process that involves removing a number of either attachment mechanisms used to hold down and tension screens, or replacing and tensioning filter mesh to an integral screen, as described above. These screen replacement operations may involve significant downtime of the sifter, which may slow the manufacturing operation. Additionally, present pre-tensioned screens are prone to structural failure do to frame breakage and loss of filter mesh tension over time.
Accordingly, there exists a continuing need for a pre-tensioned sifter screen that may resist structural failure and may provide for more efficient screen changes.